1. Field of the Invention
This invention relates to pressurized water reactors and, more particularly, to a spider, for mounting and adjustable positioning of rod clusters, having an improved vane configuration which mitigates flow-induced vibration responses while increasing the load carrying capability of the spider vanes and decreasing the manufacturing cost thereof.
2. State of the Relevant Art
As is well known in the art, conventional pressurized water reactors employ a number of control rods which are mounted within the reactor vessel, generally in parallel axial relationship, for axial translational movement in telescoping relationship with the fuel rod assemblies. The control rods contain materials known as poisons, which absorb neutrons and thereby lower the neutron flux level within the core. Adjusting the positions of the control rods relative to the respectively associated fuel rod assemblies thereby controls and regulates the reactivity and correspondingly the power output level of the reactor. Typically, the control rods, or rodlets, are arranged in clusters, and the rods cf each cluster are mounted to a common, respectively associated spider. Each spider, in turn, is connected to a respectively associated adjustment mechanism for raising or lowering the associated rod cluster.
In certain advanced designs of such pressurized water reactors, there are employed both control rod clusters (RCC) and water displacer rod clusters (WDRC), and also so-called gray rod clusters which, to the extent here relevant, are structurally identical to the RCC's and therefore both are referred to collectively hereinafter as RCC's. In one such reactor design, a total of over 2800 reactor control rods and water displacer rods are arranged in 185 clusters, each of the rod clusters having a respectively corresponding spider to which the rods of the cluster are individually mounted.
In the exemplary such advanced design pressurized water reactor, there are provided, at successively higher, axially aligned elevations within the reactor vessel, a lower barrel assembly, an inner barrel assembly, and a calandria, each of generally cylindrical configuration, and an upper closure dome. The lower barrel assembly has mounted therein, in parallel axial relationship, a plurality of fuel rod assemblies which are supported at the lower and upper ends thereof, respectively, by corresponding lower and upper core plates, the latter being welded to the bottom edges of the cylindrical sidewall of an inner barrel assembly. Within the inner barrel assembly there are mounted a large number cf rod guides disposed in closely spaced relationship, in an array extending substantially throughout the cross-sectional area of the inner barrel assembly. The rod guides are of first and second types, respectively housing therewithin reactor control rod clusters (RCC) and water displacer rod clusters (WDRC); these clusters, as received in telescoping relationship within their respectively associated guides, generally are aligned with respectively associated fuel rod assemblies.
One of the main objectives of the advanced design, pressurized water reactors to which the present invention is directed, is to achieve a significant improvement in the fuel utilization efficiency, resulting in lower, overall fuel costs. Consistent with this objective, the water displacement rodlet clusters (WDRC's) function as a mechanical moderator control, all of the WDRC's being fully inserted into association with the fuel rod assemblies, and thus into the reactor core, when initiating a new fuel cycle. Typically, a fuel cycle is of approximately 18 months, following which the fuel must be replaced. As the excess reactivity level diminishes over the cycle, the WDRC's are progressively, in groups, withdrawn from the core so as to enable the reactor to maintain the same reactivity level, even though the reactivity level of the fuel rod assemblies is reducing due to dissipation over time. Conversely, the control rod clusters are moved, again in axial translation and thus telescoping relationship relatively to the respectively associated fuel rod assemblies, for control of the reactivity and correspondingly the power output level of the reactor on a continuing basis, for example in response to load demands, in a manner analogous to conventional reactor control operations.
The calandria includes a lower calandria plate and an upper calandria plate. The rod guides are secured in position at the lower and upper ends thereof, respectively, to the upper core plate and the lower calandria plate. Within the calandria and extending between aligned apertures in the lower and upper plates thereof is mounted a plurality of calandria tubes in parallel axial relationship, respectively aligned with the rod guides. A number of flow holes are provided in remaining portions of the calandria plates, at positions displaced from the apertures associated with the calandria tubes, through which the reactor core outlet flow passes as it exists from its upward passage through the inner barrel assembly. The core outlet flow, or a major portion thereof, turns from the axial flow direction to a radial direction for passage through radially outwardly oriented outlet nozzles which are in fluid communication with the calandria.
In similar parallel axial and aligned relationship, the calandria tubes are joined to corresponding flow shrouds which extend to a predetermined elevation within the dome, and which in turn are in alignment with and in close proximity to corresponding head extensions which pass through the structural wall of the dome and carry, on their free ends at &.he exterior of and vertically above the dome, corresponding adjustment mechanisms, as above noted. The adjustment mechanisms have corresponding drive rods which extend through the respective head extensions, flow shrouds, and calandria tubes and are connected to the respectively associated spiders to which the clusters of RCC rods and WDRC rods are mounted, and serve to adjust their elevational positions within the inner barrel assembly and, correspondingly, the level to which the rods are lowered into the lower barrel assembly and thus into association with the fuel rod assemblies therein, thereby to control the reactivity within the core.
A critical design criterion of such reactors is to mitigate vibration of the reactor internals structures, as may be induced by the core outlet flow as it passes through the reactor internal structures. A significant factor for achieving that criterion is to maintain the core outlet flow or an axial direction throughout the inner barrel assembly and thus in parallel axial relationship relatively to the rod clusters and associated rod guides. This is achieved, in part, by the location of the water inlet and outlet nozzles at an elevation corresponding approximately to that of the calandria assembly, and thus above the inner barrel assembly which houses the rod guides and associated rod clusters, as above noted. Additionally, structural elements known as formers are included within the vessel to assist in maintaining the desired axial flow condition within the inner barrel assembly, in accordance with the invention disclosed in the copending application entitled "MODULAR FORMER FOR INNER BARREL ASSEMBLY OF PRESSURIZED WATER REACTOR"--Gillett et al., Ser. No. 798,195 filed Nov. 14, 1985 and assigned to the common assignee hereof now U.S. Pat. No. 4,752,441, issued June 21, 1988.
It has been determined, however, that the conventional configuration of rod cluster spiders renders them susceptible to flow-induced vibrations, even though the desired axial flow condition is maintained. Such a circumstance is of extreme concern, since vibrations accelerate the rate of wear of the internals structural elements, including particularly the rods and associated, supporting structures, leading to shortened life of these structural components and increased maintenance expense in the operation of the reactor.
Conventional reactor designs do not incorporate a reactor coolant flow path that results in the spider vanes being subjected to the bulk flow field; instead, the flow paths generally direct the coolant flow radially outwardly (i.e., from an axial path) prior to the flow reaching the normal elevation, or axial operating position, of the spiders. Accordingly, conventional reactor internals have no structural analogy to the dense packing of rod guides and associated rod clusters as are employed in advanced reactor designs of the type herein contemplated nor do they have any similar flow path requirements as exist therein, and thus they do not present the critical design concerns relating to flow-induced vibration of the spider vanes, as above explained. Thus, there are no known solutions to these problems, consistent with the structural and operational requirements of, and taking into account the environmental factors which exist in, advanced design reactors as hereinabove set forth.